There are a number of other free software projects to provide a Linux experience for mobile devices. Some of these projects are actively working on moving to Halium. All of them are kindred spirits in the quest for Linux on mobile. Not the least, they have valuable resources available at their respective sites about porting to mobile devices.
LuneOS is based on Open webOS. It is provided by the WebOS Ports team. The system has it's legacy in projects by LG, HP and Palm. There are ongoing conversations about a cooperation with Halium.
Plasma-mobile is based on the Plasma shell, developed by the KDE community. Plasma mobile is available on Halium.
PostmarketOS is based on Alpine Linux. It has a selection of multiple phone interfaces. A main difference to Halium is that postmarketOS targets upstream kernels. This brings the advantage of more up to date software. It has the disadvantage that device drivers which are only available as binaries for old kernels can not be used.
Sailfish OS is based on the Mer Project. The user interface is developed by Jolla and not open source. The technical architecture should be largely compatible with Halium.
Ubuntu Touch is based on Unity 8 and is developed by UBports. It is the continuation of the earlier Ubuntu Touch project from Canonical. UBports is actively working on making their future releases available on Halium.
There are more projects that share similar goals and/or similar tools and might have valuable resources. A few shout outs: LineageOS, MaruOS, Neo900, Purism Librem 5, Replicant, Tizen.